Current aircraft rely on electric, pneumatic and hydraulic systems for secondary power. A typical electric system utilizes an integrated drive generator (IDG) coupled to each engine to provide fixed frequency 400 Hz power to the distribution system and loads. Alternatively, a generator may be directly coupled to each aircraft engine and the variable frequency power developed by the generator(s) may be converted into fixed frequency power by a power converter. In either case, a 28 volt DC system may further be provided for some loads. The pneumatic system utilizes bleed air from the main engine compressors to provide pneumatic power for de-icing, environmental control and cabin pressure, main engine start and air driven hydraulic pumps. The hydraulic system consists of main engine driven pumps, electrically driven pumps and pneumatically driven pumps. An auxiliary power unit (APU) typically provides pneumatic, electrical and, occasionally, hydraulic power.
While the foregoing systems are useful to provide secondary power as required by aircraft loads, such systems are not jointly optimized to reduce size, weight and cost. Particularly, the hydraulic system is poorly sized with respect to speed and flow requirements. Hydraulic flow requirements typically are greatest when engine speeds are low and lessen at high cruise speeds. These requirements, however, are inconsistent with the flow handling capabilities of engine-driven pumps, which develop highest flow rates only at high engine speeds. Attempting to shift hydraulic loads to electrically driven pumps is not a good solution because the motors for such pumps usually create inrush current problems for the electrical system. Air driven pumps could be used, but they are large and inefficient.
Air turbine start equipment is large, heavy, expensive and located in the tightly packaged engine nacelle. This equipment is used only one or two minutes per flight and is thereafter merely extra weight which detracts from the load handling capability of the aircraft.
The aircraft electrical system provides high quality power to all loads, including those that do not need it. Galley equipment and solid state driven pumps are examples of those loads that do not require high quality power.
Ground engine start requires the APU to be running or a pneumatic ground cart which adds to the expense of airport operations.